The disclosure relates generally to steam turbines, and more particularly, to a flow path for an outer casing of a steam turbine.
Steam turbines often are very large in size and consequently have large material mass. Steam turbines also operate at high temperatures that create a number of challenges. One challenge is to ensure proper thermal response of parts, such as an outer casing. Typically, outer casings of steam turbines are not provided with any special thermal response system other than to provide some steam leakage and specific stage steam conditions. These thermal response techniques, however, use higher temperature steam. One approach to provide better thermal response has been to position the outer casing exhaust port at the middle of the lower half of the outer casing. Unfortunately, this configuration does not impact the region of the outer casing that drives clearances.
Another challenge is to provide an appropriate amount of clearance between outer and inner casings to avoid contact therebetween caused by the differential thermal expansion in parts thereof as they increase to the high operating temperatures. Most steam turbines address the differential thermal expansion by providing sufficient clearance between casing parts to handle any worst-case situation. This latter approach, however, increases machine size and may increase machine material mass. Another approach to the clearance issue has been to use heating blankets to bring the outer casing up to temperature before startup.